This invention relates to apparatus for reproducing, from a transmission of pulses a rectangular waves form of pulses as would be produced by the action of opening and closing a pure resistive circuit having a source of constant potential.
In telephone and telegraph circuits, of pure resistance, the action of keying, of lifting and restoring the receiver or hand set, and of dialing, would produce a clean rectangular wave shape. However, such circuits are practically never pure resistance, because the lines have inductance in various forms, such as loading coils, and capacity, such as that between wires of a cable, and between the wires and ground. Therefore instead of obtaining rectangular waves which would be produced in a circuit of pure resistance the opening and closing operation of the actual circuits results in pulses having oscillatory characteristics.
Attempts to record waves produced in the above manner, by ink recorders, from telephone, telegraph, and teletype circuits, may produce a tape which is more or less difficult to read, or may even be completely unintelligible.
In translating transmitted signal pulses as may be encountered in telephone, telegraph and teletype circuits into pulses of rectangular form suitable for such recording, and must first intercept the transmitted signal pulses and pass then through. This differentiates the waves, as will be understood. Then we may pass them through a filter or integrating circuit to remove any undesired components, such as ringing current. Next, we may amplify the resultant as and to the extent desired.
Then, we may pass them through back-to-back rectifiers to convert them to waves always having the same sign. The unidirectional pulses so resulting may then be impressed on the input circuit of a so-called one-shot multivibrator, to produce rectangular waves or pulses, the duration of each pulse so produced being somewhat less than that of a dial pulse at the fastest possible dialing speed as small as the closest possible dialing speed.
Because the multivibrator produces one rectangular pulse for each pulse supplied to its input circuit, and because each dial pulse, when differentiated by line or circuit action, gives rise to two spikes, one positive for the leading edge and one negative for the trailing edge, the multivibrator gives out two rectangular pulses for each dial pulse. The first multivibrator pulse has its leading edge synchronized with the leading edge of the dial pulse or positive spike, and the second multivibrator pulse has its leading edge synchronized with the trailing edge of the dial pulse, or negative spike.
It may be noted at this point that such occurrences as phone pick-up and hang-up also give rise to positive and negative pulses respectively, but the time between them may be very long, compared with the time between the leading and trailing edges of the dial pulses, so that pick-up and hang-up may be regarded as independent events, each producing only one pulse. The trailing edge of the clockwise dialing pulse may be regarded as being synchronous or in coincidence with the leading edge of the first dial pulse, so the clockwise dialing pulse may also be regarded as an independent event, giving rise to only an pulse subsequent to the pick-up pulse, and of the same sign, but of smaller amplitude when differentiated.
From the foregoing, it will be clear that the multi-vibrator produces a rectangular pulse the leading edge of which is synchronous with the leading edge of the pick-up pulse, another rectangular pulse the leading edge of which is synchronous with the leading edge of the clockwise dialing pulse, and two rectangular pulses for each dial pulse, synchronous with the leading and trailing edges thereof, respectively.
These rectangular pulses from the multivibrator may then pass through a differentiation circuit which produces, by differentiation, two spikes of opposite sign for each rectangular pulse. In one embodiment of the invention, a negative spike is synchronous with the leading edge of the pick-up pulse, the leading edge of the clockwise dialing pulse, and with the leading and trailing edges of each dial pulse.
Positive spikes follow each negative spike, at a constant time interval, less than the duration of any pulse to be recorded, but such positive spikes are not synchronous with either leading or trailing edge of any pulse to be recorded, and are not utilized for recording. The negative spikes, on the other hand, are synchronous with the leading edge of the pick-up and clockwise dialing pulses, and with the leading and trailing edge of each dial pulse and may be used eventually to control a pen recorder. For convenience of reference these negative spikes may be called marker spikes, since they mark the beginning and end of each dial pulse. Successive pairs of marker spikes mark dialing pulses and spacing pulses respectively, and they may be readily distinguished, whatever speed of dialing may be involved, because the duration of dialing and spacing pulses is different, whatever the dialing speed, one always being longer than the other.
These spikes may then be supplied to a frequency divider or xe2x80x9cflip-flopxe2x80x9d circuit, having two conditions of equilibrium, and driven from one to the other by application of a voltage pulse to its control. By adjusting this frequency divider circuit so that it does not respond to the positive spikes, but only to the negative, the frequency divider acts as an on-off switch, changing its state each time it receives a negative spike, and thereby producing a current which flows only during dial pulses, and which can be utilized to operate a relay controlling the pen recorder.
The frequency divider also produces a current which flows during pick-up and clockwise dialing pulses, and which starts synchronously with the leading edge of each, continues for a duration longer than the duration of dial pulses, but is interrupted an appreciable time later, but before the end of the pick-up or clockwise dialing pulse, in a manner to be described. This permits the recorder to show pick-up and clockwise dialing, but prevents the possibility of confusing such pulses with dial pulses.
Because pick-up and clockwise dialing pulses each produce only one negative spike at the frequency divider, as compared to two for each dial pulse, some means must be provided for producing and supplying to the frequency divider a synthetic negative pulse after the negative pulse marking the beginning of the pick-up and clockwise dialing pulses, otherwise the frequency divider will be thrown out of step. This is done by a space return circuit in which a condenser is allowed to charge at a constant rate, and reaches a value which will operate a discharge circuit in a time which is shorter than the shortest pick-up and clockwise dialing pulse, but longer than the longest possible dialing pulse. This discharge generates a negative spike which occurs a predetermined time after the beginning of the pick-up and clockwise dialing pulse and serves synthetically to mark the end, although the end actually has not occurred, and may therefore be said to anticipate the end of said pulses.
An additional circuit may be provided for locking in a circuit activity indicator, responsive to operation of the frequency divider, which may advise the operator that the circuit being monitored is in operation. This indicator may be a lamp, which will remain lighted until the operator actuates a manual reset switch, which he may do when monitoring of the circuit shows that it has ceased operation. To facilitate monitoring, a separate amplifier with a jack may be operated from the input transformer secondary.
In a modification of the invention, in case it is desired not to record pick-up and clockwise dialing pulses, and talking voltages, which appear before and after the dial pulses, an additional tube and associated apparatus may be connected to the multi-vibrator and frequency divider which suppress all but the dial pulses.
In still another embodiment of the invention, we may employ a circuit which is basically similar to that already described, but differing therefrom in that it is polarized, i.e., the input terminals must be connected to the line in the correct sense.
The features of novelty which we believe to be characteristic of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to its fundamental principles and as to its particular embodiments will best be understood by reference to the specification and accompanying drawing, wherein we have described our invention, and the manner and process of making and using it, and have set forth the best mode contemplated by us of carrying it out.
As will be understood from the foregoing, a principal object of this invention is to translate transmitted signal pulses of a separate signal circuit into pulses of rectangular wave form suitable for recording.
Another object of this invention is to reproduce pulses of rectangular wave forms, recreated from a separate signal circuit and suppress or eliminate undesired. circuit occurrences.